1. Field of the Invention
The present invention relates to focus adjusting apparatuses and focus adjusting methods usable in image pickup apparatuses.
2. Description of the Related Art
Conventionally, in a case where automatic focusing (AF) is performed in electronic still cameras and video cameras, a method is used, in which a lens position at which a high frequency component of a luminance signal supplied from an image-pickup element, such as CCD (Charge Coupled Device), is maximum is treated as an in-focus position. The following scan method is known as such a method. In the scan method, an evaluation value (focus evaluation value) calculated based on the high frequency component of the luminance signal obtained from the image-pickup element is successively stored while the lens is driven over its entire move range, and the lens position at which the thus-stored evaluation value exhibits the maximum is treated as the in-focus position.
In another method, a lens continues to be moved in such a direction that the focus evaluation value increases. This method is known as the hill climbing method (also referred to as continuous AF hereinafter).
Further, Japanese Patent No. 4,106,485 discloses the following method. In this method, the continuous AF is executed prior to instructions for image-taking preparation operation to maintain an in-focus state, thereby restricting the move range of a focus lens to be moved by the AF scan method for performing the image-taking preparation operation subsequent to instructions therefor. Thus, the AF operation time is decreased.
In general, in electronic still cameras and the like, a diaphragm is arranged in an image-taking lens to adjust the amount of light input from an object. Further, automatic exposure (AE) control is generally performed prior to instructions for the image-taking preparation operation so that an object image formed in an electronic view finder can have an appropriate brightness. In this AE operation, the diaphragm is controlled.
When the diaphragm is changed, a depth of focus changes. When the depth of focus changes, a focus state varies even if the position of the focus lens remains unchanged. When the diaphragm moves due to the AE operation during the AF scan and the depth of focus changes, the focus evaluation value obtained when the diaphragm is opened cannot be correlated with that obtained when the diaphragm is somewhat closed. Even if the focus adjustment is conducted using the focus evaluation value obtained without considering the state of the diaphragm, precision in the focus adjustment is likely to lower.
Further, an ND (Neutral Density) filter can be used in place of the diaphragm. In electronic still cameras, a lens unit can include plural lenses. In such a case, the ND filter can be interposed between the plural lenses or between the lens and the image-pickup element, and removably inserted into an optical path. The optical path length at the time the ND filter is inserted into the optical path is different from that at the time the ND filter is removed from the optical path. Therefore, even when the position of the focus lens remains unchanged in those states with and without the ND filter, the focus state varies therebetween. Also in the case of the ND filter, when the ND filter is inserted or removed due to the AE operation during the AF scan, the focus evaluation values cannot be correlated with each other for the same reason described above. Thus, precision in the focus adjustment likewise lowers.